1. Field of the Invention
This invention relates to nickel hydroxide particles which can be used suitably as a starting material for a lithium nickel composite oxide (lithium nickelate) as a cathode (positive electrode) active material for a non-aqueous electrolyte lithium ion secondary battery, a method for production of the nickel hydroxide particles, lithium nickel composite oxide particles produced using the nickel hydroxide particles as a starting material and a method for production of the composite oxide particles.
2. Description of the Prior Art
Along with popularization of portable electronic appliances in recent years, non-aqueous electrolyte lithium ion secondary batteries having high energy density and capable of providing high voltage have attracted attention. As a cathode active material for 4V class non-aqueous electrolyte lithium ion secondary batteries, there have been known composite oxides of lithium and transition metals having layered or tunnel structures and having crystal structures allowing easy doping and dedoping of lithium ions, such as lithium cobaltate, lithium nickelate or lithium manganate. Among these composite oxides, lithium cobaltate has a problem that the raw material, cobalt is produced at limited areas and is very expensive as well as the supply thereof lacks stability. On the other hand, lithium manganate has a problem in that high energy density as in the case of lithium cobaltate can not be obtained although the material cost can be reduced relatively.
In the meanwhile, lithium nickelate has been considered promising in that nickel resource is abundant and it has satisfactory capacity characteristics compared with the above-mentioned two composite oxides, and, in addition, it can realize the highest energy density of the three. Further, there have also been known composite oxides based on lithium nickelate such as LiNi.sub.1-X M.sub.X O.sub.2 (in which M represents magnesium, calcium, strontium, barium, aluminum, cobalt, manganese, iron or vanadium, and X is a number satisfying: 0&lt;X&lt;1) or LiNi.sub.1-X-Y Co.sub.X M.sub.Y O.sub.2 (in which M represents aluminum, magnesium, calcium, strontium or barium, and X and Y each represents a number satisfying: 0&lt;X&lt;1, 0&lt;Y&lt;1 and 0&lt;X+Y&lt;1). These composite oxides have been positively developed heretofore since they have excellent cell characteristics, that is, they have a high charge discharge capacity, can provide high voltage, and have excellent cycle characteristics, as well as that the raw material for nickel is relatively less expensive and is supplied stably.
Such composite oxides as mentioned above can be obtained usually by dry mixing a nickel salt containing the metal M described above (and cobalt salt) or a nickel salt (and cobalt salt) and a salt of the metal M with a lithium compound, or wet mixing them in an appropriate solvent, drying and then sintering the resulting mixture at a temperature usually from 600.degree. C. to 1000.degree. C. for 10 to 30 hours in an oxidative atmosphere and, optionally, applying pulverization and classification.
In the method for the production of the composite oxides as described above, hydroxides, oxides, carbonates, nitrates or sulfates can be used for the nickel salt as the starting material. However, in fact, nickel hydroxide has been mainly used so far since it is produced industrially, inexpensive, stable in quality and gives less problems related to public pollution derived from gases generated from the sintering process.
However, nickel hydroxide used so far as the raw material for the production of the composite oxides is available as secondary particles of a particle diameter of about 5 to 30 .mu.m comprised of agglomerates of a primary particles of particle diameter of about 0.1 .mu.m Japanese Patent Laid-Open (Kokai) No. 230808/1995 recommends the use of spherical particles of a particle diameter of about 5 to 50 .mu.m comprised of agglomerates of primary particles of a particle diameter of 0.1 .mu.m or less. However, lithium nickelate obtained by using the known nickel hydroxide as the starting material is comprised of agglomerates of small primary particles of a particle diameter of 1 .mu.m or less.
On the other hand, Iithium nickelate known so far as a cathode active material for non-a(queous electrolyte lithium ion secondary batteries has a relatively high charge discharge capacity, but there remains a problem unsolved that, in view of practical use of lithium nickelate as a cathode active material, the capacity decreases when charge discharge cycles are conducted at a high temperature of about 45.degree. C. or, self-discharge takes place when stored at a high temperature. As described in Japanese Patent Laid-Open No. 151988/1993 and No. 183047/1995, the problems result from the size of primary particles of lithium nickelate. That is, it is considered that they are caused, for example, because the reaction of lithium nickelate with the non-aqueous electrolyte, or the decomposition of electrolyte and the formation of films occur more remarkably on the boundaries of the lithium nickelate particles as the size of the primary particles thereof is smaller.
Then, with a view point that non-aqueous electrolyte lithium ion secondary batteries using the composite oxides such as lithium cobaltate, lithium nickelate or lithium manganate as a cathode active material are excellent in cycle characteristics or storage characteristics as described above, it has been pointed out that the composite oxide used, for example, lithium cobaltate, preferably has an average particle diameter (50%) of 2 to 10 .mu.m in order to prevent the decrease of the capacity when charge discharge process is repeated (Japanese Patent Laid-Open No. 94822/1993). Further, it has been pointed that the decrease of the capacity less occurs even when charge discharge process is repeated at a high temperature when a composite oxide such as lithium cobaltate or lithium nickelate has such a particle size distribution that 10% accumulated diameter is 3 to 15 .mu.m, 50% accumulated diameter is 8 to 35 .mu.m and 90% accumulated diameter is 30 to 80 .mu.m (Japanese Patent Laid-Open No. 151998/1993). It has also been pointed out for lithium manganese that the average particle size is preferably within a range from 30 to 100 .mu.m (Japanese Patent Laid-Open No. 283074/1993).
On the other hand, it has been pointed out that a non-aqueous electrolyte secondary battery using a cathode active material comprising lithium manganese composite oxide has excellent cycle characteristics when the lithium manganese composite oxide has a specific surface area in the range of 0.05 to 5.0 m.sup.2 /g (Japanese Patent Laid-Open No. 69790/1996).
However, nickel hydroxide that forms lithium nickelate particles having a large primary particle size has not been known heretofore,
The invention has been accomplished in view of the foregoing situations in the known non-aqueous electrolyte lithium ion secondary batteries, in particular, the known cathode active material. Therefore, it is an object of the invention to provide nickel hydroxide secondary particles comprised of agglomerates of primary particles having a large primary particle diameter which can be used suitably for the production of a cathode active material for lithium ion secondary batteries, method for production of such nickel hydroxide secondary particles, lithium nickel composite oxide particles produced using the nickel hydroxide particles as the starting material, as well as a method for production of such composite oxide particles.